1. Field of the Invention
This invention is in the field of non-destructive testing by means of magnetic particle inspection. It seeks to provide a composition and a method which enable the indications to be held more firmly to the workpiece through the medium of including a water-sensitive binder material in the magnetic particle composition which is actuated by an aqueous spray to cause firm adherance of the indications to the workpiece.
2. Description of the Prior Art
Testing of magnetizable workpieces by means of magnetic particle inspection techniques is a highly developed art (see, for example, Metals Handbook, Eighth Edition, Volume 11, pages 44 to 74).
More recently, the magnetic particles have been combined with fluorescent particles either in the form of a water or oil suspension whereupon the workpiece was inspected under filtered ultraviolet or black light to observe any concentration pattern of fluorescent particles caused by a surface discontinuity. Prior art patents referring to this type of inspection technique include Switzer U.S. Pat. No. 2,267,999 and Kazenas U.S. Pat. No. 2,936,287. These patents relate, respectively, to lacquer bonded and resin bonded fluorescent magnetic particles for use in this type of inspection.
Methods of making fluorescent coated magnetic particles have also been described in U.S. Pat. Nos. 3,404,093 and 3,485,758, assigned to the assignee of the present application.
Fluorescent magnetic particle inspection is becoming increasingly important in the inspection of steel billets. Various defects can be detected in steel billets by using this inspection process. Discontinuities such as arrowhead cracks and longitudinal cracks are relatively easy to observe. Seams in the billet, however, pose a more serious problem. Seams are longitudinal discontinuities that appear as light lines in the surface of the steel. Normal seams are similar to longitudinal cracks, but produce lighter indications. Seams normally are closed tight enough that no actual opening can be visually detected without a magnetic particle inspection. Seams have a large number of possible origins, some mechanical and some metallurgical. Brush seams are clusters of short seams that give the appearance of having been painted or brushed onto the surface. Usually, these defects are the result of removal of metal from the steel surface by scarfing or scaling, exposing ingot blowholes and subsurface porosity. They may range in depth from about 0.005 to 0.300 inch and may occur either in zones or across the entire surface of the billet.
Laps are longitudinal discontinuities of varying severity that are caused by formation of ribs or extensions of metal during hot rolling and the subsequent folding over of these protrusions. Laps usually run at acute angles to the surface. Frequently, they occur at opposite sides of the billet, and very often run over the entire length of the billet.
Scabs appear as extraneous pieces of metal partially welded to the surface of a steel billet. The two major sources of scabs are splashing of metal against the mold wall during teeming, and adherence of scarfing wash or fins to blooms after conditioning.
One of the problems in conducting magnetic particle inspection of steel billets arises from the fact that the billets are subjected to relatively rough handling from the time the fluorescent magnetic particles are applied to the surface to the time they are inspected. Frequently, the handling is severe enough so that clusters of magnetic particles at a defect are broken loose from the surface of the workpiece and are not detected at the inspection station. Consequently, the inability of the indications to stay on the imperfection reduces the reliability of the test procedure.